In general purification treatment such as hemodialysis treatment, a blood circuit including flexible tubing is used to place patient's blood in extracorporeal circulation. This blood circuit arrangement mainly includes an arterial blood circuit having an arterial needle at one end to collect blood from the patient and a venous blood circuit having a venous needle at one end to return the blood to the patient. A dialyzer between the arterial blood circuit and the venous blood circuit purifies the blood circulating extracorporeally.
Such a dialyzer is composed of plural hollow fibers inside the dialyzer. Blood flow inside each hollow fiber and the flow of dialysate outside the hollow fibers (i.e. between an external surface of the hollow fiber and an inside surface of a dialyzer case). The hollow fiber has a blood purification membrane with small pores on its surface. Wastes in the blood flowing inside the hollow fiber is discharged into the dialysate after passing the blood purification membrane and the blood, without the wastes, is returned to the patient. Also, a ultrafiltration pump installed inside the dialyzer to remove water from the patient's blood allows the removal of water during dialysis.
When a needle is detached from a patient to whom the needle had been attached during a hemodialysis treatment due to any reason, and especially when the arterial blood needle is detached from the patient, air is indrawn from the arterial blood needle and an air-lock is generated in the dialyzer. Also, when the venous blood needle is detached from the venous blood circuit, the patient's blood introduced from the arterial blood needle is wasted without returning to the patient. According to such failure, the pressure (venous blood pressure) of patient's blood flowing in the venous blood circuit is monitored to detect if the needle has dropped out. Specifically, when the needle is detached from the patient, the needle is open to the air and the blood pressure flowing in the venous blood circuit decreases so that the dropping out of the needle from the patient can be detected by detecting the blood pressure.
Normally the blood pressure (venous blood pressure), which is measured by a pressure sensor, changes depending on a number of factors, e.g., clogging in the dialyzer or a change of the condition of patient's blood during the hemodialysis treatment, so that the venous blood pressure is changeable even under normal conditions. Therefore, it is a drawback that judgment about whether the change of the pressure is due to a change of patient's position, which is normal, or due to a detachment of the venous blood needle, which is abnormal, cannot be carried out correctly.
Based on such reasoning, for example, as disclosed in Japanese Patent Publication No. 2003-518413, a hemodialysis device has voltage adding means (the voltage generator in the patent) for adding a voltage to the blood flowing in the blood circuit by adding a voltage between two electrodes, measuring means (the detector in the patent) for measuring a voltage difference at two positions according to the voltage added by the voltage adding means, and monitoring means (the calculation means in the patent) for obtaining the voltage difference by comparing the values measured by two measuring means. Specifically, as the measuring means is attached, the voltage difference obtained by the monitoring means varies when the needle is detached from the patient, and the detachment of the needle can be monitored effectively based on such change.